The present invention relates generally to amplifiers, and more particularly to high frequency wide bandwidth amplifiers.
In the wireless communications industry, a premium is placed on the ability to amplify wide bandwidth signals, e.g., spread spectrum signals, in a highly efficient manner. As an example, a typical eighteen-channel base station requires approximately 540 watts of RF power output (30 watts per each channel). Assuming a typical power amplifier efficiency of 5 percent, the amount of power needed to generate an RF power output of 540 watts will be 10.8 kW, with 10.26 kW being dissipated as heat. This dissipated heat represents a drawback in that it not only requires the use of fans and heat sinks to cool the base station, but also translates to wasted energy, thereby reducing battery life. In short, the cost of the base station increases as the efficiency of the power amplifiers used in the base station decreases.
Although various attempts have been made to address this problem, it remains difficult to design a high efficiency power amplifier that is able to linearly amplify wide bandwidth signals. This is due to the paradoxical nature of a typical amplifier, which exhibits a wide bandwidth capability that is inversely proportional to its efficiency. The amplification of spread spectrum signals, such as code division multiple access (CDMA) signals, which typically have high peak-to-average signal amplitude ratios, make it difficult, if not impossible, to continuously operate the power amplifier in saturation, thereby reducing the efficiency of a power amplifier even further.
A method that has been proposed to solve this problem involves the use of envelope elimination and restoration (EER), which is a technique through which highly efficient radio frequency (RF) power amplifiers can be combined to produce a high efficiency linear amplifier system. In this method, a modulated input signal is split into two paths: an amplitude path through which the envelope of the modulated input signal is processed, and a phase path through which the phase modulated carrier of the modulated input signal is processed. The envelope of the modulated input signal is amplified through a highly efficient amplifier, which operates at the narrower modulated bandwidth, i.e., the bandwidth of the envelope, thereby producing an amplified envelope signal. A high frequency, highly efficient amplifier is then used to modulate the high frequency phase modulated carrier with the amplified envelope signal, thereby generating an amplified replica of the modulated input signal. Specifically, the amplifier that generates the amplified envelope signal acts as the DC power supply to the high frequency amplifier. The efficiency of this EER amplifier system can be calculated by multiplying the efficiencies of the two amplifiers. For example, if the efficiency of each of the amplifiers is 50%, the total efficiency of the EER amplifier system will be 25%.
Although the use of an EER amplifier system to amplify wide bandwidth modulated signals is, in general, beneficial, its efficiency and maximum modulation bandwidth is dependent upon the efficiency and bandwidth of the power supply amplifier.
Accordingly, it would be desirable to provide apparatus and methods for increasing the efficiency and bandwidth of an amplifier for a variety of purposes, which may include the provision of DC power to an RF power amplifier within an EER amplifier system.
The present invention is directed to systems and methods for efficiently amplifying relatively wide bandwidth signals.
In accordance with a first aspect of the present inventions, a method can be employed to amplify an input signal (e.g., an envelope signal obtained from a CDMA signal), which exhibits one or more lower frequency components and one or more higher frequency components. It should be understood that xe2x80x9clowerxe2x80x9d and xe2x80x9chigherxe2x80x9d are relative terms, and are only meant to define a frequency component with respect to another frequency component. It should also be understood that a lower frequency component may encompass a DC component. The method includes generating a first signal by amplifying the input signal, sensing the first signal (e.g, the current), and generating a second signal by amplifying the power residing in the one or more lower frequency components of the first signal. The power residing in the one or more lower frequency components of the first signal is inversely varied with the second signal. In effect, amplification of the power residing in the one or more lower frequency components of the input signal is minimized during the first stated amplification, and maximized during the second stated amplification. The first and second signals are then combined to produce an amplified input signal. The positive feedback provided from the sensed first signal to the amplification of the second signal provides stability to the amplified input signal. Optionally, the second signal can be sensed (e.g., the voltage) and the input signal inversely varied with the sensed second signal, e.g., to minimize noise otherwise created during the second stated amplification.
The bandwidth of the input signal may be relatively wide, e.g., between 0 MHz and 10 MHz. In such a case, the one or more lower frequency components will make up a relatively narrow range within the bandwidth, e.g., between 0 MHz and 1 MHz, and the one or more higher frequency components will make up a relatively higher range within the bandwidth, e.g., between 1 MHz and 10 MHz. In such a case, amplification of the one or more high frequency components is preferably accomplished at a first power efficiency, and amplification of the one or more low frequency components is preferably accomplished at a second higher power efficiency. As a result, the power within the narrower low frequency range is efficiently amplified, and the power within the wider high frequency range is amplified substantially free of distortion.
In accordance with a second aspect of the present inventions, an amplifier circuit is provided for amplifying an input signal, e.g., an envelope signal extracted from a CDMA signal. The amplifier circuit includes an AB-type amplifier, which is configured to receive the input signal, and a synchronous buck DC/DC converter, the input of which is coupled to the output of the amplifier through a positive feedback loop. A resistive load is coupled in parallel between the respective outputs of the AB-type amplifier and the DC/DC converter. In the preferred embodiment, the positive feedback loop includes a current sensor coupled to the output of the AB-type amplifier, and a pulse width modulator coupled between the current sensor and the input of the DC/DC converter. The sensed current is driven to a relatively low value due to the feedback process, such that mostly the higher frequency components remain in the sensed current. Optionally, a negative feedback loop can be coupled between the output of the DC/DC converter and the input of the AB-type amplifier. The negative feedback loop may include a differential operational amplifier having an output coupled to the input of the AB-type amplifier, an inverting input coupled to the output of the converter, and a noninverting input for receiving the input signal.
In accordance with a third aspect of the present inventions, an amplifier circuit is provided for amplifying an input signal. The amplifier circuit comprises a dependent voltage source, e.g., an AB-type RF amplifier, that operates at a first bandwidth and a first power efficiency, and a dependent current source, e.g., a synchronous buck DC/DC converter, that operates at a second bandwidth narrower than the first bandwidth and a second power efficiency greater than the first power efficiency. The dependent voltage source is configured to generate a first voltage that varies with the input signal, and the dependent current source is configured to generate a second current that varies with a first current produced by the dependent voltage source. A load is coupled in parallel between the dependent voltage source and the dependent current source, wherein the first voltage appears across the load, and the first and second current flow through the load.
Optionally, the dependent voltage source is configured, such that the first voltage inversely varies with a second noise voltage generated by the current source. In the preferred embodiment, the first bandwidth encompasses the second bandwidth, with the second bandwidth being at the lower end of the first bandwidth. For example, the first bandwidth can range from 0 MHz to 10 MHz, with the second bandwidth ranging from 0 MHz to 1 MHz.
Other objects and features of the present invention will become apparent from consideration of the following description, taken in conjunction with the accompanying drawings.